U.S. patent application number 13/639241 was filed with the patent office on 2013-02-21 for treatment of a heater tube intended for a pressurizer of the primary cooling system of a nuclear reactor.
This patent application is currently assigned to ELECTRICITE DE FRANCE. The applicant listed for this patent is Jacques Champredonde, Jean-Marie Fageon, Yves Neau. Invention is credited to Jacques Champredonde, Jean-Marie Fageon, Yves Neau.
Application Number | 20130044852 13/639241 |
Document ID | / |
Family ID | 43127096 |
Filed Date | 2013-02-21 |
United States Patent
Application |
20130044852 |
Kind Code |
A1 |
Champredonde; Jacques ; et
al. |
February 21, 2013 |
TREATMENT OF A HEATER TUBE INTENDED FOR A PRESSURIZER OF THE
PRIMARY COOLING SYSTEM OF A NUCLEAR REACTOR
Abstract
A treatment of a heater tube intended to be used in a
pressurizer of the primary cooling system of a nuclear reactor. In
particular, the heater tube comprises a heater housed in a
substantially cylindrical sheath. The material of which this sheath
is made is a work-hardened austenitic stainless steel. In
particular, the external surface of the sheath is liable to undergo
a stress corrosion during use of the heatertube. The method
includes a heat treatment step, preferably using induction heating,
in which the external surface of the sheath is heat-treated so as
to recrystallize the material of the sheath at least on the surface
thereof.
Inventors: |
Champredonde; Jacques;
(Antony, FR) ; Fageon; Jean-Marie; (Cergy, FR)
; Neau; Yves; (Montigny sur Loing, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Champredonde; Jacques
Fageon; Jean-Marie
Neau; Yves |
Antony
Cergy
Montigny sur Loing |
|
FR
FR
FR |
|
|
Assignee: |
ELECTRICITE DE FRANCE
Paris
FR
|
Family ID: |
43127096 |
Appl. No.: |
13/639241 |
Filed: |
April 6, 2011 |
PCT Filed: |
April 6, 2011 |
PCT NO: |
PCT/FR2011/050775 |
371 Date: |
October 4, 2012 |
Current U.S.
Class: |
376/361 ;
148/570; 148/590 |
Current CPC
Class: |
C21D 1/09 20130101; Y02P
10/25 20151101; C21D 9/0068 20130101; G21C 17/017 20130101; C21D
2201/03 20130101; H05B 3/42 20130101; Y02E 30/30 20130101; C21D
1/10 20130101; G21C 1/09 20130101; C21D 7/06 20130101 |
Class at
Publication: |
376/361 ;
148/590; 148/570 |
International
Class: |
G21C 1/09 20060101
G21C001/09; C21D 1/42 20060101 C21D001/42; C21D 9/08 20060101
C21D009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 8, 2010 |
FR |
1052651 |
Claims
1. A method for the treatment of a heater tube intended to be used
in a pressurizer of the primary cooling system of a nuclear
reactor, the heater tube including a heater housed in a
substantially cylindrical sheath, said sheath including an external
surface that is liable to undergo stress corrosion, at least
partially, during the use of the tube, said sheath including a
steel-type material, wherein the method comprises a heat treatment
step of the external surface at least of said sheath for
recrystallizing the material of the sheath, at least on the
surface.
2. The method according to claim 1, wherein the heat treatment uses
induction heating applied starting from the external surface of the
sheath.
3. The method according to claim 2, wherein the heat treatment
includes a rise in temperature on the external surface of the
sheath comprised within a range from 900.degree. C. to
1,050.degree. C.
4. The method according to claim 3, wherein the rise in temperature
of the heater resulting from the heat treatment is limited to a
maximum value of 900.degree. C.
5. The method according to claim 3, wherein an alternating current
frequency applied in an inductor coil for recrystallization
treatment using induction heating is at least 100 kHz, for a coil
30 to 50 mm in diameter surrounding the external surface of the
sheath, the diameter of the sheath being of the order of 20 to 25
mm.
6. The method according to claim 3, wherein an inductor is arranged
around the tube, and in that a relative displacement of the
inductor with respect to the tube is applied, at least in
translation along the tube.
7. The method according to claim 6, wherein the speed of the
translational displacement is comprised between 100 and 900 mm per
minute, for a power supplied by induction comprised between 1 and
50 kW.
8. The method according to claim 2, wherein the heat treatment
implements an inductor of the solenoid type arranged around the
tube.
9. The method according to claim 1, further comprising a supply of
inert gas onto the external surface of the sheath in order to avoid
oxidation following the heat treatment.
10. The method according to claim 1, further comprising, after the
heat treatment, a step of cooling by blowing fluid onto the
external surface of the sheath.
11. The method according to claim 1, wherein the external surface
of the sheath has at least traces of work-hardening before the heat
treatment step.
12. The method according to claim 1, wherein the material of the
sheath is of the work-hardened austenitic stainless steel type.
13. A heater tube produced by the method according to claim 1,
wherein the sheath of the tube includes at least on its external
surface a thickness of recrystallized material.
14. The tube according to claim 13, wherein the thickness is
greater than or of the order of 1 mm.
15. The tube according to claim 13, wherein it has a hardness
equivalent to a value less than or equal to approximately 240
Vickers.
Description
FIELD OF THE INVENTION
[0001] The invention relates to the heater tubes for a pressurizer
of a primary cooling system of a pressurized water nuclear
reactor.
[0002] STATE OF THE ART
[0003] A heater tube, for such a pressurizer of a primary cooling
system, normally comprises an outer metal casing that has an
elongated cylindrical shape (for example 22 mm diameter by 2 m long
approximately) called a "sheath", and a heater mounted inside the
sheath.
[0004] Such tubes are mounted on a lower portion of the
pressurizer, as explained in detail in document FR-2 895 206, and
are submerged in the water of the primary cooling system that
contains the pressurizer. They are used to raise the operating
pressure of the primary cooling system. It will thus be understood
that the tubes are under load when in use and undergo, in
particular, thermal stress which, when combined with work-hardening
stress as described below, potentially causes stress corrosion.
Problems Posed
[0005] Incidents have shown that leaks could occur on the heater
tubes of the state of the art. In particular, the sheath of a tube
can crack, such that the inside of the tube is open to the water
present in the pressurizer. There follows a possible deterioration
of the heater of the tube, loss of operation of the tube, or even
the leaking of the pressurized water to the outside of the
pressurizer, through the inner space of the tube.
[0006] As a result, a solution is sought for limiting the risks of
cracking of the sheath, due in particular to the stress corrosion
that the sheath undergoes.
[0007] A solution was proposed in the above-mentioned document FR-2
895 206 that aimed to deposit a protective nickel coating onto the
external surface of the sheath, using electrolysis. However, the
solution of adding material leads to an alteration in the geometry
of the tube, in particular by increasing the diameter thereof.
Moreover, the solution is not completely secure, as the risk of
removal of the nickel layer under the effect of impacts or friction
cannot be ruled out. Given the increase in diameter, this solution
cannot be used with heater tubes that are already manufactured, as
they may no longer match the dimensions of the supports.
Furthermore, it is lengthy to implement.
[0008] The present invention aims to improve the situation.
General Presentation of the Invention
[0009] To this end the invention proposes the treatment of the
tubes with a view to reducing the above-mentioned cracking risks.
The treatment provided in the context of the invention is,
generally, the heat treatment of the tubes in order to
recrystallize at least the external surface of the sheath.
[0010] Thus, the present invention relates to a method for the
treatment of a heater tube intended for use in a pressurizer of the
primary cooling system of a nuclear reactor. The heater tube
includes a heater housed in a substantially cylindrical sheath. The
sheath includes an external surface that is liable to undergo
stress corrosion, at least partially, while the tube is in use.
[0011] In particular, as the sheath includes a steel-type material,
for example of the work-hardened austenitic stainless steel type,
the method in the context of the invention includes a heat
treatment step of at least the external surface of the sheath, in
order to recrystallize the material of the sheath, at least on the
surface thereof.
[0012] The material thus recrystallized is not subject to the
phenomenon of stress corrosion by comparison with the tubes of the
state of the art, without deterioration, which eliminates the risks
of cracking and ultimately extends the life of the tube.
[0013] Preferably, the heat treatment uses induction heating
applied starting from the external surface of the sheath.
[0014] In particular, a heat treatment is envisaged that includes a
rise in temperature on the external surface of the sheath comprised
within a range of 800.degree. C. to 1,100.degree. C. and preferably
between 900.degree. C. and 1,050.degree. C. or between 950.degree.
C. and 1,050.degree. C., for example 960.degree. C., 970.degree. C.
or even 1,000.degree. C.
[0015] By applying a heat treatment using induction heating, the
rise in temperature of the heater resulting from the heat treatment
is advantageously limited to a maximum value of 900.degree. C.,
allowing the electrical resistance and isolation properties of the
heater to be retained.
[0016] In an embodiment described in detail hereinafter, the heat
treatment using induction heating consists of applying an
alternating current in the windings of an inductance coil
surrounding the external surface of the sheath. The frequency of
the alternating current can be chosen and is preferably at least
100 kHz. The higher the frequency, the more the energy transmitted
to the sheath using induction heating is concentrated on a small
thickness of the sheath according to a so-called "skin" effect.
Said frequency value is given in a context where the induction
winding has a diameter of 30 to 50 mm and for a sheath the diameter
of which is of the order of 20 to 25 mm.
[0017] The inductor is arranged around the tube and, in particular,
a relative displacement of the inductor with respect to the tube is
preferably applied, at least in translation along the tube.
[0018] In one embodiment, the speed of the translational
displacement is comprised between 100 and 900 mm per minute, for a
power supplied by induction comprised between 1 and 50 kW.
[0019] Preferably, the inductor is of the solenoid type.
[0020] In one embodiment, a supply of inert gas can moreover be
provided onto the external surface of the sheath in order to avoid
oxidation following the heat treatment.
[0021] After the heat treatment, it is also possible to apply
cooling by blowing a fluid (for example air) onto the external
surface of the sheath.
[0022] The present invention also relates to a heater tube, as
such, obtained by the method in the context of the invention. In
particular, the sheath of the tube includes at least on its
external surface a thickness of recrystallized material. The
thickness is preferably greater than or of the order of 1 mm. The
thickness is advantageously comprised between approximately 1 mm
and a total thickness of the sheath of the tube, and more
particularly comprised between approximately 1.5 mm and
approximately 3 mm, for example approximately 2 mm.
[0023] By "recrystallized material" is meant the fact that the heat
treatment applied contributes to regenerating severely deformed
grains having high hardness, into grains with equal axes having
high or medium hardness. Thus, a trace of the method of the
invention on the tube consists in that the hardness of the sheath
on its external surface is lower than for a standard tube of the
state of the art. Typically, a hardness equivalent to a value less
than or equal to approximately 240 Vickers or even less than
approximately 200 Vickers can be measured on the external surface
of the sheath of a treated tube in the context of the invention.
These hardness values represent respectively recrystallized
material thicknesses greater than or of the order of 1 mm or
approximately 1.5 mm to 2 mm.
[0024] As explained hereinafter, initially, the heater is mounted
in the sheath of the tube by crimping, the external surface of the
sheath being swaged. Work-hardening of the external surface of the
sheath results. As will be seen hereinafter, there is a synergic
effect between the work-hardening and the heat treatment in the
context of the invention.
[0025] It is then possible to observe on a tube, before the heat
treatment in the context of the invention, traces of work-hardening
by swaging, in particular on the external surface of the sheath.
Advantageously, the consequences of the work-hardening (in
particular in terms of stress corrosion resistance) disappear
overall after the treatment of the invention.
Advantages Provided by the Invention
[0026] Thus, the heat treatment chosen in the context of the
invention is preferably a treatment using induction heating, aiming
to promote recrystallization of the material from which the sheath
is made, in particular on the external surface of the sheath. By
way of non-limitative example, the material of the sheath can
typically be an austenitic steel (containing essentially iron, 16
to 20% chrome and 8 to 14% nickel, as well as carbon (less than 1%)
and optionally molybdenum, niobium or titanium).
[0027] It has in fact been observed that the risk of corrosion of
the sheath of a tube can be linked to its method of manufacture by
swaging, causing the substantial work-hardening of the metal, in
particular on the external surface of the sheath. FIG. 3 represents
an enlarged view of the surface SUR of the sheath of a tube,
showing in particular very work-hardened grains close to the
external surface SUR of the sheath.
[0028] For this first reason, heat treatment using induction
heating is advantageous since, in principle, firstly it promotes a
rise in temperature in particular on the external surface of the
material treated using induction heating.
[0029] Treatment using induction heating is also advantageous at
least for a second reason: it is suspected that overall heat
treatment (at approximately 1,050.degree. C. for recrystallizing
the sheath of a tube) might cause deterioration of the electrical
properties of the tube and in particular of the heater mounted
inside the sheath. As a result, surface heat treatment of the tube
only, and in particular of the sheath, selectively, is preferred in
one embodiment of the invention. Heat treatment using induction
heating is therefore suitable. When the temperature of the heater
is above 900.degree. C., it is in fact suspected that deterioration
of the electrical properties may occur.
[0030] Thus, treatment using induction heating, advantageously of
the surface of the sheath, makes it possible to improve the
morphological defects (significant plasticization, dislocations and
local stresses) on the surface of the sheath, linked in particular
to the work-hardening of the sheath during the manufacture of the
tube.
[0031] Moreover, when the heat treatment is carried out by means of
a solenoid surrounding the tube, the recrystallization heat
treatment can be implemented without creating any heat treatment
discontinuities.
[0032] Axially, continuous and regular heat treatment can be
obtained by continuous and regular displacement of the tube in the
inductor, or vice-versa.
[0033] Radially, heat treatment takes place simultaneously over the
whole circumference of the sheath with substantially equal
intensity. The risks of forming radial stress non-uniformity during
the recrystallization treatment are therefore low.
[0034] In particular, the stresses due to the work-hardening of the
sheath during manufacture of the tube are absorbed uniformly over
the circumference of the tube.
[0035] Stress non-uniformity could occur if, during the surface
heat treatment, certain areas of the sheath that are more
significantly work-hardened undergo recrystallization treatment to
a lesser extent than other areas of the tube sheath that are less
significantly work-hardened. Radial stress non-uniformity creates
areas of high stress on one side of the tube and areas of low
stress on another side of the tube, which could contribute to
bending the tube.
[0036] Moreover, the energy (therefore the temperature) required
for recrystallizing a work-hardened steel is less than for a steel
that is not work-hardened. For example, while a non work-hardened
steel starts recrystallization at 1,050.degree. C., the same steel
superficially work-hardened needs only a smaller rise in
temperature, for example 960.degree. C., considering moreover that
not all of the surface of said steel is work-hardened and that the
work-hardening is not homogeneous over the whole thickness of the
sheath. This observation makes it possible to reduce the
temperature to be applied to the sheath for its recrystallization
and therefore also to reduce the temperature that the heater must
undergo inside the sheath.
[0037] Use of a surface temperature comprised between 900.degree.
C. and 1,050.degree. C. or more particularly between 950.degree. C.
and 1,050.degree. C., for example 960.degree. C., 970.degree. C. or
even 1,000.degree. C. makes it possible to ensure surface
recrystallization when the surface of the sheath includes areas
that are less significantly work-hardened than other areas. In
particular, these surface temperatures make it possible to
recrystallize portions of the sheath that are less work-hardened
than the external surface, for example areas closer to the
centre.
[0038] As mentioned previously, there is a synergic effect between
the work-hardening and the heat treatment in the context of the
invention. In particular, the work-hardening initially present
makes it possible to reduce the temperature of the treatment.
Moreover, the treatment according to the invention makes it
possible to overcome defects from the manufacturing of the tubes by
work-hardening. The heat treatment according to the invention
allows the majority of the stresses present in the sheath to be
absorbed, including residual stresses caused by the work-hardening
and present deep within the sheath, below the external surface.
[0039] When the recrystallization treatment is carried out over a
thickness of the order of those mentioned above, in particular
approximately 1.5 mm or approximately 2 mm, the majority of the
thickness of the sheath is treated. The majority of the stresses
induced in the sheath by work-hardening during the manufacture of
the tube are then absorbed. The external surface of the sheath thus
undergoes only minimal stress on the part of layers that are
further inside the sheath.
[0040] By absorbing the stresses due to the work-hardening of the
sheath, the method according to the invention makes it possible to
reduce the stresses that are present overall in the tube to values
less than approximately 100 MPa, or even less than approximately 80
MPa. Thus, the stresses present overall in the tube are markedly
less than the limit stresses above which stress corrosion can take
place in use, i.e. for tubes having a sheath made from austenitic
steels, stresses of the order of 300 MPa to 400 MPa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] Other features and advantages of the invention will become
apparent on reading the detailed description below of
non-limitative examples, as well as examining the attached drawings
in which:
[0042] FIG. 1 shows a cross-sectional view of a tube, showing in
particular the inside of a tube in the context of the
invention;
[0043] FIG. 2 shows a detail of FIG. 1, with in particular
different points at which the temperature associated with the
treatment using induction heating is estimated according to the
graphs in FIGS. 4 and 5;
[0044] FIG. 3 shows a microscopic view of the surface of the sheath
of a tube;
[0045] FIG. 4 is a graph showing estimated temperature profiles
according to time at different points of the tube, the detail of
which is shown in FIG. 2, under conditions of treatment using
induction heating at a frequency of 300 kHz, with a double-coil
2,000-ampere inductor and stopping heating at 4.6 seconds, not
maintained;
[0046] FIG. 5 is a graph showing estimated temperature profiles at
different points of the tube, the detail of which is shown in FIG.
2, under conditions of treatment using induction heating at a
frequency of 200 kHz, with a double-coil 3,000-ampere inductor, not
maintained;
[0047] FIG. 6 gives a very diagrammatic representation of an
installation for implementing the method.
DESCRIPTION OF EMBODIMENTS
[0048] Firstly, reference is made to FIG. 1, in which the portion
of the tube intended to be submerged in a pressurizer is shown. In
this case, it includes a cylindrical-shaped sheath 5 made from
stainless steel. It will thus be understood that the method can be
applied to any tube the sheath of which is produced from the
general family of "stainless steels" (without particular limitation
of the proportion of alloys forming said steel).
[0049] The central core of the tube includes a mandrel 2, usually
made from copper, inside the sheath 5, along the central axis of
the sheath, as well as a heating wire 1 coiled around the mandrel 2
in a spiral and interposed between the mandrel 2 and the sheath 5.
The heating wire constitutes the heater mentioned above in the
general presentation of the invention.
[0050] The heating wire 1 comprises an electrically conductive
resistive metal core 3, for example made from copper or
nickel-chrome alloy. A protective metal coating made from steel 6
(see in particular the detail in FIG. 2) surrounds the core 3. The
coating 6 is electrically isolated from the core 3 by an insulator
4 for example magnesia (MgO). The heating wire 1, wound around the
mandrel 2 forming contiguous turns, is intended to be connected to
a connector electrically connected to an electricity generator
making an electric current flow in the conductor wire 1. Details on
the connection of such a heater tube and its use in the primary
cooling system of a nuclear reactor are described in publication
FR-2 895 206.
[0051] Referring now to FIG. 2, the thickness of the sheath 5
(between points A and C), in an embodiment that is in no way
limitative, is 2.45 mm. The thickness of the protective coating 6
of the heating wire 1 is 0.5 mm (between points C and D in FIG. 2).
The thickness of the magnesia lining 4 is 0.4 mm (between points D
and E in FIG. 2). Thus it will be understood that the
representation in FIGS. 1 and 2 is not necessarily to scale.
Finally, the diameter of the conductive core 3 of the heating wire
is approximately 1.5 mm (between points E and F).
[0052] Furthermore, the elements surrounded by the sheath 5 are
crimped into the sheath according to a step of shrinking the sheath
by swaging, which moreover generates the mechanical stress that is
liable to affect the stress corrosion resistance. After shrinking,
the sheath 5 is in close contact with the coils 1 of the heating
element, as shown in particular in FIG. 2.
[0053] According to a first series of tests carried out, a rise in
temperature of the external surface of the sheath 5 of
approximately 1,050.degree. C. was sought, for the purpose of its
recrystallization. With reference to FIG. 4, it was estimated that
the external surface of the sheath (curve A) exhibited a
temperature rise peak of 1,050.degree. C., promoting
recrystallization. At point J, corresponding to approximately 83%
of the power received by induction ("skin effect" known in
treatment using induction heating), the rise in temperature is
approximately 1,000.degree. C. In particular, curve B shows the
temperature profile at 1.5 mm from the external surface of the
sheath (at point B in FIG. 2). It became apparent that a rise in
temperature to only 900.degree. C. already allowed
recrystallization of the material of the sheath. Thus, said first
series of tests made it possible to recrystallize practically the
whole of the sheath, including its volume. It will be observed
however, on the curve marked E, that the temperature of the core 3
of the heating wire does not exceed 800.degree. C., making it
possible to retain the conductive properties of the core 3 of the
heating wire, thus ensuring that the treatment in the context of
the invention does not produce any deterioration of the content of
the tube. Overall, a rise in temperature of the external surface of
the sheath is sought within a range of 800.degree. C. to
1,100.degree. C., and preferably 900.degree. C. to 1,050.degree.
C., a temperature range sufficient to recrystallize the material of
the sheath. To said constraint is added a maximum rise in
temperature of the magnesia 4 that is limited to 850.degree. C. (at
point D in FIG. 2), in order to ensure a smaller rise in
temperature of the core 3 of the heating wire.
[0054] In order to respect these constraints, advantageously a set
of induction parameters is chosen from at least: [0055] the
frequency f(Hz) of the alternating current flowing in the coils of
the inductor (reference IND in FIG. 6), it being understood that
the higher said frequency, the more the energy received by
induction is confined to the surface of the sheath 5 (by skin
effect), [0056] the power P (W) or as an equivalent the amperage of
the current for the chosen frequency, [0057] the duration of
application of the heat treatment, shown in the example in FIG. 6
by a speed V (mm/min) of relative displacement of the inductor IND
with respect to the sheath 5 of the tube.
[0058] Of course, the lower the speed of the inductor with respect
to the tube, the greater the rise in temperature.
[0059] These different effects are thus shown in FIG. 5, which
represents an estimate of the temperature rises for a higher speed
of travel, but with a higher power density. It will be noted here
that the interface between the protective coating of the heating
wire and the magnesia (point D) undergoes a rise in temperature of
less than 750.degree. C.
[0060] According to the set of tests carried out, it transpires
that the frequency of the alternating current to be provided is
preferably greater than 150 kHz, so as to protect the magnesia 4
and/or the conductive core 3 of the heating wire 1, while limiting
the rise in temperature to a threshold value of the order of 800 to
900.degree. C. The power supplied can be within a range of 1 to 50
kW. The relative speed of movement of the inductor IND with respect
to the tube can be comprised within a range of 100 to 900 mm/min.
Under these conditions, it is preferable to provide a solenoid
inductor having an inside diameter of 30 to 50 mm, it being
understood that the diameter of the tube, in a given embodiment, is
22 mm.
[0061] Preferably, as shown in FIG. 6, the tube is rotated during
heat treatment (arrow R) about its central axis, in order to
homogenize the heat treatment applied to the sheath.
[0062] Of course, the parameters of the treatment using induction
heating such as, in particular, the frequency, the power and the
speed of travel are adjustable in the treatment installation shown
in FIG. 6 according to the precise dimensions of the elements
constituting the tube, according to their material, or other
constraints. It will be understood generally that the effect sought
in the treatment using induction heating is to create an
alternating magnetic field (using alternating currents flowing in
the inductor) in order to generate induced currents on the external
surface of the sheath of the tube. Said induced currents instantly
heat the area where they occur. On the other hand, the inner
elements of the tube such as the inner surface of the sheath, and
in particular the heating wire 1 and the mandrel 2 are, in
principle, only heated by thermal conduction (as clearly shown by
curves E to I in FIGS. 4 and 5). It will thus be understood that
the treatment thickness is ultimately a function of the chosen
frequency value (for the skin effect) and of the treatment time, or
in an equivalent manner, of the speed of travel of the inductor
with respect to the tube (by thermal conduction).
[0063] Recrystallization of at least the external surface of the
sheath 5 of the tube then occurs. The recrystallization is seen in
particular by the fact that the material becomes softer when
recrystallized. Typically it is possible to measure a hardness of
less than or equal to approximately 240 Vickers by a penetration
measurement using a conical diamond at a pressure of 5 kg on the
external surface of the sheath 5 of a tube treated using the method
in the context of the invention. The thickness of the
recrystallized sheath is at least 1 mm.
[0064] Thus it will be understood that tracing the method in the
context of the invention on the treated tube consists of measuring
a hardness less than or equal to approximately 240 Vickers, for
example over at least 1 mm thickness from the external surface of
the sheath 5 of the tube.
[0065] FIG. 6 shows the blowing B of a fluid onto the tube,
immediately after the treatment using induction heating. Indeed a
cooling effect can be provided (for example by air) in order to
reduce the temperature of the elements constituting the tube, after
recrystallization of the sheath. In this way the temperature is
reduced at the ends of the curve, as shown in FIGS. 4 and 5.
[0066] The tube can also be protected from oxidation (after rise in
temperature) by installing a muffle (quartz sleeve around the tube)
for supplying an inert gas (for example argon, helium or possibly
nitrogen). Said muffle supplying an inert gas (not shown in FIG. 6)
can operate between the inductor IND and the air blower B in the
diagram shown.
[0067] In a variant, the heat treatment can be carried out in a
cabinet under an inert gas atmosphere in order to avoid superficial
oxidation of the sheath.
[0068] More generally, the present invention is not restricted to
the embodiments given above; it extends to other variants.
[0069] Thus, the air blower B shown in FIG. 6 for cooling the tube
can simply be removed.
[0070] Moreover, the application of inert gas onto the sheath is
also optional. Due to the short duration of treatment, the possible
oxidation of the tube remains limited. At most, a slight blueing of
the external surface of the sheath 5 is noted. Said oxidation can
simply be removed by a final pickling step (a step already planned
and implemented in the general manufacturing method of the tubes).
During said pickling step, the thin oxidation layer formed by the
treatment using induction heating is removed, making it possible to
avoid providing for the blowing of inert gas or applying the heat
treatment in an inert gas chamber such as described above.
[0071] Moreover, as stated above, the temperature rise values given
in the examples in FIGS. 4 and 5 allow numerous variants.
Generally, it can be assumed that as the recrystallization of the
sheath can take place between 800 and 1100.degree. C., the
conditions of treatment using induction heating aim to raise the
temperature of the external surface of the sheath accordingly,
while seeking to limit the rise in temperature of the heating wire
to approximately 900.degree. C. at most. Moreover, it is also
preferable that the rise in temperature of the external surface of
the sheath does not exceed a threshold value, for example above
1,100.degree. C., or that the duration of the heat treatment is
also limited to a threshold value, in order not to promote
so-called "secondary recrystallization" which is seen overall
through a lack of homogeneity in the size of the crystalline
grains, weakening the material.
[0072] Moreover, as explained above, if the external surface of the
sheath is work-hardened overall, the maximum temperature rise at
the surface of the sheath (peak of curve A of FIG. 4 or 5) can be
reduced below 1000.degree. C., for example to 960.degree. C.
[0073] More generally, heat treatment using induction heating has
been described above by way of example, but the invention can be
applied to any type of heat treatment capable of selectively
restricting the rise in temperature mainly to the sheath of the
tube. For example, heating by laser scanning or by annular torch on
the surface of the sheath can be envisaged. The treatment by
annular torch, reproducing heat treatment having similar advantages
to those of treatment by a cylindrical solenoid, is particularly
advantageous.
* * * * *